Hey guys I recently built a mud motor using the Mad Mud motor plans, with a few modifications. The motor works and moves great. I am having one problem with it and was wondering if any one else had the same problem. I used a 9hp motor for the build. When the motor is running it makes a very bad vibrating noise as soon as the ONLY the prop is in the water. The cav plate is above the water, it seems as soon as the prop touches the water it makes the noise. When I start the motor with the boat on the trailer and have the motor set a mid-range RPM it still makes the noise. I replaced the u-joint and both yolks. All three bushings were checked and appear to be fine. Im about to replace the shaft and see if that fixes that problem. When I threaded the shaft, the threaded part has high spots and low spots, it doesnt look like a piece of all-thread or a bolt. Im not sure why it threaded this way. The prop threaded over the low spots fine and was tight on the high spots. I have a video of the motor but MMT will not allow me to upload it. Anyone have ideas on what it might be? Im about to tear the whole thing down and rebuild it if I cant figure it out. I can email the video if anyone is interested. Thanks

Can you post pics of the threaded portion of the shaft? It's possible that the shaft isn't balanced properly causing a wobble and the vibrations.

Did you thread the shaft yourself by hand? Shaft might be threaded crooked, this would cause vibrations. Best to have a machine shop thread shafts, to guarantee it is straight and centered. Possibly shaft is bent also, but from explanation of threads for propeller, seems that is your problem.

Yeah I threaded the shaft by hand and it was a BITCH to do, its very possibly I messed them up when cutting them. Im gonna hit up the steel store and machine shop today.

motor prolly has about 1 hour of deep water use on it, never hit a rock on even seen mud yet.....

Replaced shaft and motor works like it should. I do believe the prop was in a low spot on the threads and causing the vibration........ Now to find some mud!
Thanks guys

Good to hear

sweet! Let us know how it runs.

Ya your shaft, the threaded section, you need to take it to a machine shop. Those treads look real bad.